Traditional steel elevator ropes can be visually inspected for wear. The individual wires will break and these breaks can be easily observed. Aramid fiber elevator ropes are typically covered with a protective sheathing which makes visual inspection difficult. Even if the ropes were not sheathed, it would still be difficult to determine the proper time to replace the ropes because the appearance of the fibers is almost identical whether the fibers are new or in need of replacement.
Prior art ropes have resorted to placing conductive fibers within the ropes so that the fibers can be monitored by electrical means. For example, U.S. Pat. No. 5,834,942 to De Angelis, issued Nov. 10, 1998, discloses an apparatus for determining when a synthetic fiber cable for an elevator is ready for replacement. The apparatus includes a voltage detection device for detecting a voltage in at least one carbon fiber of the synthetic fiber cable and at least one threshold device for determining when the detected voltage exceeds a predetermined voltage threshold. The detected voltage depends upon the integrity of that portion of the synthetic cable in which the carbon fibers are located. Exceeding the predetermined voltage threshold is indicative of a failure of that portion of the cable. This apparatus, therefore, may not be suitable for synthetic cables that are not readily conductive.
It has been shown that the elastic properties of aramid materials can be determined from the measurement of wave propagation through the material. (See M. Ferreira et al., “Nondestructive Testing of Polyaramide Cables by Longitudinal Wave Propagation: Study of the Dynamic Modulus”, Polymer Engineering and Science, Vol. 40, No. 7, July 2000, which is hereby incorporated by reference in its entirety.) In particular, it has been observed that polyaramid cables at different states of fatigue have their own speed of longitudinal propagation of acoustic waves. It has been observed that longitudinal waves travel through aramid fiber ropes in accordance with the following formula:                               V          2                =                  E          ρ                                    (                  Equation          ⁢                                           ⁢          1                )            where V=velocity of wave propagation, E=dynamic or sonic modulus, and ρ=density. Since the tensile modulus and acoustic modulus both change at the same rate with fatigue, it is possible to calculate the tensile modulus from the observed values of wave propagation. Plotting E (modulus) against Fr (residual strength), it was found that E=f (Fr). In other words, a quantifiable relationship exists between modulus (determined from velocity) and residual strength.
A similar relationship between modulus and residual strength may be determined for aramid ropes used in elevator systems. The relationship will vary based on the particular aramid material used and the dimensions of the rope. Once the relationship is determined, it will be possible to extrapolate the residual strength of the rope from determinations of modulus. This has not heretofore been achieved for elevator systems.
Thus, an objective of the present invention is to provide an apparatus and method for inspecting aramid fiber elevator ropes which are under tension, and for calculating the residual strength of such ropes to determine when they need replacement.